1. Field of the Invention
The invention concerns a method for improving the quality of printing with a thermotransfer print head and an arrangement for implementation of the method. The invention is used in printing devices with relative movement between the thermotransfer print head and the print good, in particular in franking machines and in accounting or mail processing apparatuses that print in a similar manner. The invention is more specifically for increasing quality in the printing of data matrix barcodes with a high throughput of mail pieces, particularly for improving the machine-readability of such data matrix barcodes.
2. Description of the Prior Art
A franking machine with a thermotransfer print device that more easily allows changing of the print image information is described in U.S. Pat. No. 4,746,234. Semi-permanent and variable print image information are electronically stored as print data in a memory and are read out in the thermotransfer print device for printout thereof. As is generally known, the print image (franking stamp image) includes identification and postal information, including the postal fee data for conveyance of the mail piece, for example a postage value image, a postal image with the postal delivery location and date, as well as an advertising stamp image.
The entire print image is printed by a single thermotransfer print head in print image columns controlled by a microprocessor-controlled. The printing of the print columns ensues orthogonally relative to the transport direction on a moving mail piece. A typical machine of this type can achieve a maximum throughput of franking items of 2200 letters/hour at a print resolution of 203 dpi.
The franking machine T1000, commercially available from Francotyp-Postalia GmbH, has only one microprocessor for controlling a thermotransfer print head with 240 heating elements in printing in columns. All heating elements lie in a row which is 30 mm long and is arranged orthogonal to the transport direction. For printing, thermotransfer printers use an at least equally wide thermotransfer ink band which is arranged between a surface to be printed (for example of a mail item) and the series of heating elements. At the resistor of the activated heating element the energy of an electrical pulse is transduced into heat energy which transfers to the thermotransfer ink ribbon. Printing requires melting a small area of an ink layer from the thermotransfer ink ribbon and application of the melted ink layer onto the print good surface. The printing ensues only if the heating element charged with the pulse was brought to printing temperature, i.e. to a temperature higher than the preheating temperature Given movement of the thermotransfer ink ribbon together with the mail item relative to the heating elements and given a running heat energy feed, a line (dash) is printed in one row parallel to the movement (transport) direction. A line is printed in a print column orthogonal to the movement or transport direction when all heating elements in the row of heating elements are simultaneously charged with electrical pulses for a predetermined, limited time duration (pulse duration). The pulse duration can be sub-divided into phases. Within the predetermined, limited time duration (pulse duration), a last phase (print phase) exists in which the dots of a print column are printed. Further phases of the activation of the heating elements precede the last phase in order to heat the printing element to the printing temperature. Print image columns also can be associated with these phases due to the transport of the mail piece. A longer individual pulse for activation of a heating element can be divided into a number of pulses whose pulse durations are identical and correspond to a specific heating phase. Print image columns of the moving mail item are thus likewise associated with these heating phases, as the print columns are associated with the print phases.
The binary pixel data for activation of the heating elements of all print columns are non-permanently stored in a pixel memory. Given a low print resolution, the spacing of adjacent print columns is large and the binary pixel data of the print phase reflect the print image. A number of pulses are conventionally required in order to generate sufficient heat energy for melting an area of the ink layer under the heating element, the ink layer area then being printed as a dot on the surface of the mail piece (DE 38 33 746 A1).
In principle, to achieve a high print resolution printing could ensue in each phase when the activation of the heating elements for heating thereof ensues only in a timely manner in preceding phases. This requires that the energy of an electrical pulse is likewise transduced into heat energy at the resistor of the adjacent heating element in the row (heat conduction problem). The heat energy is reduced by cooling when the pulse is omitted. Due to the adjacent energy application, spread of heat energy by heat conduction can be taken into account by the activation of specific heating elements for heating thereof being interrupted in one phase, but nevertheless sufficient heat energy is present to effect melting of the ink layer area under the heating element. A microprocessor is therefore also programmed to control the energy distribution dependent on the pattern to be printed, in addition to the preparation and output of binary pixel data for generation or non-generation of an electrical pulse. The original representation of the print image by binary pixel data is thus correspondingly altered in the pixel memory so that a cleaner print image is created. This requires either a comprehensive preliminary calculation (as is, among other things, known from EP 53 526 B1 (=DE 41 33 207 A1) Method for Controlling the Feed of a Thermoprinting Heating Element) or a history-based control (history control). In the case of history control, the supplied energy for preheating a respective heating element of the thermotransfer print head is adjusted dependent on whether printing processes have been initiated frequently or rarely in the recent past involving activation of that heating element.
From JP 61-239966 it is known to separately control the temperature of the individual heating elements by a pulse width modulation dependent on adjacent data, and to temporarily raise the temperature to the value necessary for printing. Nevertheless, the appertaining heating element (and thus the entire thermotransfer print head) remains relatively cool in spite of the preheating. This is desirable so that the temperature curve falls off relatively steeply, so that the time between the successive raster points in time can be short. This technique shortens the time necessary for a plotting of dots on a print medium and thus increases the printing speed.
A microprocessor with a higher calculation speed could be used to achieve a higher print resolution. The output of binary pixel data to the thermotransfer print head would then ensue more often per time unit in which a mail piece or similar print item is further moved an identical amount along the transport path. The memory space requirement in the pixel memory for the pixel data, however, increases for each additionally-inserted virtual column or heating phase. A “virtual column” means the presence of a further column in the print image that is not visible upon printing since no dot is printed in the heating phase.
Since the market introduction of the franking machine T1000 (the T1000 franking machine being the first to be equipped to change the aforementioned advertisement stamp image electronically at the press of a button in addition to changing the date and the postal fees), the demands on the microprocessor controller of the T1000 franking machine have become steadily greater. More data are processed as more variable data are required in the print image. Moreover, it is also applicable to generate other print images that differ significantly from a franking stamp image in terms of design and content in order, for example, to print out business cards, fees, and court cost stamp images. The requirements for the print resolution in dpi (dots per inch) steadily increase. Upon printing of a dot, the aforementioned heat conduction problem between the adjacent heating elements due to the adjacent pixels in the print image to be printed occurs more strongly the closer that the pixels are to each other. The aforementioned problem which is connected with the thermotransfer printing method increases at high print resolution.
Modern franking machines should enable the printing of a security imprint, i.e. an imprint of a special marking in addition to the aforementioned information. For example, a message authentication code or a signature is generated from the aforementioned information and then a character string or a barcode is formed as a marking. When a security imprint is printed with such a marking, that enables a review of the authenticity of the security imprint, for example at the post office or at the private carrier (U.S. Pat. Nos. 5,953,426 and 6,041,704).
The development of the postal requirements for a security imprint in some countries has had the consequence that the amount of the variable print image data that must be changed between two imprints of different franking stamp images is very high. For example, for Canada a data matrix code of 48×48 image elements should be generated and printed for every single franking imprint.
For more rational postal distribution and to increase security against counterfeiting, a new standard called FRANKIT® was introduced in Germany by Deutsche Post AG in 2004. Even at low print speed, the print quality of known franking machines with thermotransfer printing is not good enough for the machine readability of a 2-D barcode, as required by FRANKIT. In addition to the printing speed, however, the print resolution also had be increased to 300 dpi for printing of such a two-dimensional barcode A high throughput of mail pieces means a lower quality in the printing, in particular of data matrix barcodes, such that their machine readability is not always guaranteed. The microprocessor of a franking machine suitable for this has more data to process in a shorter time. The heat energy for printing the image elements of the franking machine should be calculated in a microprocessor-controlled manner taking into account the immediately preceding two print columns printed in the past. Such a history control is known but would now have to be expanded for the purpose of taking into account much more information in order to improve the readability of data matrix barcodes.
The printed data matrix barcode, at each of the left edge and lower edge, has a continuous line (called a 100% line) and at the right edge and upper edge has a discontinuous line composed of barcode image elements (called a 50% line because every other barcode image element is missing). Instead of being printed as a point, the barcode image elements (modules) are conventionally printed in quadratic form (FIG. 1). The high-resolution images printed with previous methods, in particular barcode images, are printed out differently at the edges than in the center and thus are not always machine-readable.